The present invention relates to a method for the discriminating of obstacles by means of a radar, and its different applications to the measurement of a liquid in a tank or vessel, or again to the elimination of unwanted or parasitic echos coming notably from the ground, for radars mounted on vehicles or controlling vehicle traffic.
The proposed method meets different constraints or limitations encountered in the above-mentioned fields of application in which precise measurements have to be made, either of distance or of Doppler frequencies, in order to compute speeds. Now, the precision of the measurements made by radar depends firstly on the inherent characteristics of the radar used (waveform, width of the major transmission lobe, frequency band used, level of the secondary lobes etc.) and secondly on the different surrounding obstacles which may constitute parasitic echos for the radar and may thus disturb the measurements.
In the particular application pertaining to the measurement of the level of a liquid contained in a tank, for example a tank of a petrol tanker, it is sought to make as precise a measurement of the level as possible in order to reliably determine the total quantity of liquid contained in the tank. The approaches that are habitually proposed consist in the use of a continuous wave radar, of the FM/CW (i.e. frequency modulated, continuous wave) type working in X band, the radar being placed above the tank and being aimed vertically at the surface constituted by the liquid. The receiver of the radar then carries out the spectral analysis of the signals received after mixing with the transmission signal, according to the known principle of the correlation radar. The drawbacks of these approaches are numerous: the beam of the radar needs to be sized so as to intercept only the surface of the liquid in the tank when the level is at its lowest, without illuminating the edges of the tank, which send back parasitic echos, thus dictating a very low level of secondary lobes. Furthermore, since the resolution in distance of a correlation radar is inversely proportional to the frequency band .DELTA.F used, this frequency band should be big enough, typically of the order of 1 GHz if precision of the order of one centimeter is desired. This leads to major requirements relating to the linearity of the transceiver in the frequency band .DELTA.F used. Finally, the spectral analysis done may necessitate the use of 1024-point FFT. All these requirements make the radar costly.
In the other applications relating to radars mounted on vehicles for the measurement, for example, of their speed of movement by Doppler effect, or else to radars designed for the control of vehicle traffic, for example on the road, the measurements made are inconveniently disturbed by parasitic echos that come notably from the ground and are received by the minor lobes. This problem is generally resolved, but inadequately so, by placing major constraints on the minor lobes.
The present invention is aimed at overcoming the above drawbacks and limitations by proposing a method for the discriminating of obstacles that can be used to reduce the costs of the radar and increase the precision of the measurements, irrespectively of the measurements made (distance, Doppler, etc.).